Bevel ring gears and pinion gears are well known and commonly used in power transmission applications. Bevel ring gears and pinion gears have various well known tooth forms including straight, spiral and hypoid types.
Known motor vehicle right angle drive axles typically employ a gear set comprising a drive bevel pinion and a driven bevel ring gear. These gear sets are typically of the hypoid or spiral type. Gear sets for motor vehicle right angle drive axles must be manufactured to a high degree of accuracy and finish for smooth and quiet operation. The design and operation of these gears is well known in the prior art.
Hypoid bevel ring gears may be undercut, which means that when viewing the face of the gear in the direction of the gear axis it is not possible to see all the tooth flanks or roots. Forging undercut gears is difficult because the forged gear must be rotated as it is ejected from the die.
Conventionally bevel ring gears, in particular for vehicle right angle drive axles, are manufactured by starting with a forged blank without teeth that must be trimmed to remove considerable flash. The mounting faces, mounting holes and other surfaces except the teeth are then machined. The teeth are then cut using special gear cutting machines. The gear is then hardened and finished by “lapping” with a mating pinion. Lapping involves running the gear set under load with an abrasive paste on the gear teeth. Lapping differs from machining processes such as grinding in that lapping does not use any cutting tools or wheels. Lapping improves the surface contact between the gears and the finished ring gear and pinion typically remain as a matched set. Lapping removes very little material compared with machining processes such as grinding. Lapping essentially only improves the surface finish and contact area of the gear teeth flanks, and essentially does not change the shape or precision of the gear teeth. A disadvantage of this method is that the process of cutting the teeth takes considerable time and hence a large number of expensive special gear cutting machines are required for high volume production. Also, the cutting tools have a short life and are expensive to replace or refurbish. A further disadvantage is that trimming the forged blank and cutting the teeth wastes a considerable amount of material. A similar procedure is used for manufacturing bevel pinion gears.
In order to reduce machining, attempts have been made to forge bevel ring gears. U.S. Pat. No. 4,856,167 (Sabroff et. al.) discloses a method of near net forging bevel ring gears. The method includes the steps of hot forging ring roll preforms of substantially toroidal shape, ring rolling the preforms into forging blank rings having a substantially rectangular section, then forging said rings into near net ring gear forgings. Near net shape forging leaves a machining allowance on the gear teeth and is used because the forging process described can not forge to the final dimensions that are required for hypoid bevel ring gears for vehicle drive axles. A disadvantage of this method is that the gear must be finished using a grinding process because the hot forging process described is not accurate enough to dimensionally finish the gear teeth. Further disadvantages with hot forging include the formation of scale on the surface of the forged component and decarburisation of the steel. Grinding requires expensive special purpose machines and lapping may still be required after grinding to provide the finish needed in particular for light passenger vehicles. A further disadvantage of this method is that there is still waste metal due to the trimming operation required to make the annular blanks. A further disadvantage of this method is the complex process described for producing the annular blanks by forging, trimming and ring rolling.
U.S. Pat. No. 2,713,277 (Kaul) discloses a method of near net shape cold forging bevel ring gears from steel ring blanks in a closed die. A disadvantage of this method is that using cold forging creates considerable strain hardening and residual stresses, particularly in the forged gear teeth, that are released during subsequent hardening causing excessive distortion. This means that allowance must be provided for subsequent finish grinding. A further disadvantage is that grinding using expensive special purpose machines is required to finish the teeth after hardening, and also subsequent lapping may be required to achieve the finish needed in particular for light passenger vehicles. The grinding process must be able to remove any flash generated at the ends of the teeth, which can occur due to the design of the forging apparatus disclosed in this patent. This means that the grinding process must use slower feeds and handle higher loads than would be the case if flash was not present. A disadvantage of the die apparatus disclosed is that the annular tooth die has the outer ends of the teeth open. This allows slight flash to occur due to the sliding motion between the annular tooth die and adjacent bore and high pressures of cold forging increasing the clearance between the annular tooth die and adjacent bores. This formation of flash becomes worse as the die wears. A further disadvantage of the ends of the tooth die being open is that the tooth forms in the die do not have enough lateral support and they may distort or break during forging. This is a problem when forging spiral and hypoid gears where the force on either side of the tooth during forging is not even. Further disadvantages of the ends of the tooth die being open include rapid wear of the die and limitations on shaping the ends of the teeth. A further disadvantage of this die is that is does not provide any means to release an undercut hypoid gear after forging.
Japanese patent publication No. 4-210839 in the name of Honda Motor Co., Limited, discloses a die apparatus for ejecting undercut ring gears from a die after forging. The ejection punch engages projections formed in thick flash projecting inwards from the inner diameter of the forged gear to rotate the gear as it is removed from the die. A disadvantage of this die apparatus is that the forging cavity is open so considerable flash is formed which wastes material, including the flash containing the engagement projections that is subsequently machined off. A further disadvantage with the open die is that the forging can not be controlled accurately enough to eliminate subsequent machining.
Due to the disadvantages and limitations of the prior art methods of forging ring gears as described above, the conventional method of gear cutting is still the most common method of producing bevel ring gears and pinion gears even though gear cutting is an expensive and inefficient process.
The present invention ameliorates the manufacture of bevel gears, and in particular hypoid and spiral bevel ring gears, by providing a method and apparatus for forging these gears.